1. Field of the Invention
This invention relates to a method for utilizing a sulfuric acid/peroxide liquid mixture. More particularly, it relates to a method for utilizing a sulfuric acid/peroxide liquid mixture that can be used with advantage as a method for re-utilization of a used-up sulfuric acid/peroxide liquid mixture.
2. Description of the Related Art
A sulfuric acid/peroxide liquid mixture is so far used in large quantities as a rinsing liquid for a semiconductor wafer mainly in semiconductor manufacturing processing in the form of a sulfuric acid/aqueous hydrogen peroxide or a sulfuric acid/ozone liquid mixture.
This sulfuric acid/peroxide liquid mixture is a representative rinsing solution used mainly for exfoliating a photoresist on a wafer or removing particles of metal, in particular heavy metal. This liquid mixture is used in significantly larger quantities than in the case of other rinsing solutions, such as ammonia/aqueous hydrogen peroxide, hydrochloric acid/aqueous hydrogen peroxide or dilute fluoric acid.
The result is that a large quantity of a waste liquid is generated after use. The waste solution of this liquid mixture presents the following problems in connection with disposal:
(i) If the waste liquid is mixed with water, significant heat evolution occurs (in connection with the heat of hydration of sulfuric acid);
(ii) peroxo acid (Caro""s acid) exhibiting strong oxidating power is left; and
(iii) aqueous hydrogen oxide is left such that vigorous foaming (oxygen yielding) is likely to occur as a result of impacts or mixing of impurities.
Due to the above-described problems of the heat evolution, abrupt rise in pressure caused by foaming, or to possibility of deterioration of the equipment due to oxidation, it has been retained to be difficult, in view of safety or maintenance, to transport the waste solution directly to purify it to sulfuric acid. Thus, according to a customary process, the waste solution is diluted and cooled with a large quantity of water, neutralized with an alkali such as slaked lime, agglomerated with an inorganic flocculent and with a high molecular flocculent, in this order, and finally allowed to flow into an effluent as waste water.
Since the amount of water or chemical agents, such as flocculants, are used in an amount which ultimately is tens to hundreds of that of the original sulfuric acid/peroxide liquid mixture, the processing cost, that is the equipment or maintenance cost, cost of the chemical agents or of disposal of sludge, is increased. If this is to be deemed to be unavoidable, the amount of the waste material is also increased prohibitively.
It has so far been felt that the waste sulfuric acid/peroxide liquid mixture cannot be disposed of except by causing it to flow into an effluent, without the possibility of its re-utilization.
In view that the disposal of the waste sulfuric acid/peroxide liquid mixture has many disadvantages, as described above, the present inventors have made attempts to develop a technique for efficiently re-utilizing the waste liquid and for suppressing the problem relating to its disposal. The present invention, completed in the course of these researches, is aimed to develop a technique of utilizing the sulfuric acid/peroxide liquid mixture, such as its waste liquid, and to provide a method for utilizing the waste liquid and for suppressing the problem relating to its disposal.
According to the present invention, there is provided a method for utilizing a sulfuric acid/peroxide liquid mixture including decomposing peroxide in the sulfuric acid/peroxide liquid mixture to utilize the liquid mixture as sulfuric acid.
Since the peroxide in the sulfuric acid/peroxide liquid mixture is decomposed to utilize the resulting liquid mixture as sulfuric acid, a waste liquid of the sulfuric acid/peroxide liquid mixture can be effectively re-utilized if the waste liquid is processed by this technique. This enables effective utilization of the waste liquid to suppress the problem raised in connection with disposal. The present invention, which provides an effective technique for the waste liquid of the sulfuric acid/peroxide liquid mixture in which sulfuric acid/peroxide is left as a component, can be applied to a sulfuric acid/peroxide liquid mixture other than a waste sulfuric acid/peroxide liquid mixture as a method for utilizing the sulfuric acid/peroxide liquid mixture if it is desired to utilize it as sulfuric acid.
The present invention is completed on the basis of the following information obtained by the present inventors. That is, the present inventors have conducted perseverant investigations to overcome the aforementioned problem and accordingly have found that, on adding a decomposition accelerator, such as nitric acid, to a sulfuric acid/peroxide liquid mixture, such as its waste liquid material produced from, for example, the semiconductor manufacturing process, the waste liquid material can be utilized as regenerated sulfuric acid. This finding has led to completion of the present invention.
According to the present invention, the novel method for utilizing the sulfuric acid/peroxide liquid mixture is provided whereby it is now possible to utilize the waste liquid material of the sulfuric acid/peroxide liquid mixture as sulfuric acid. The result is that the waste liquid need not be disposed of to render it possible to make effective utilization of resources and to reduce the amount of the waste material, such as effluent water. Moreover, with the present invention, the peroxide can be completely decomposed without lowering the concentration of sulfuric acid in the liquid waste material. The resultant product can be as a modifier, such as a sulfonating agent or a hydrolysis accelerator, for a variety of compounds, such as polymers. The decomposed liquid obtained on decomposing the peroxide can be used as a modifier for the waste resin material, whereby the waste material can be converted to a high added-value material to contribute to maintenance of global environment through reduction of the waste material and effective utilization of resources.
The present invention will be explained with reference to a preferred embodiment which is merely illustrative and is not intended to limit the scope of the invention.
The present invention exploits the sulfuric acid/peroxide liquid mixture as sulfuric acid by decomposing the peroxide in the sulfuric acid/peroxide liquid mixture and finds most effective application in case the sulfuric acid/peroxide liquid mixture represents a waste liquid resulting from various fields of use of the sulfuric acid/peroxide liquid mixture because the various problems caused by disposal can then be obviated.
According to the present invention, the sulfuric acid contained in the sulfuric acid/peroxide liquid mixture has a concentration not lower than 70 wt %, preferably not lower than 80 wt %. The reason is that, if the sulfuric acid concentration is less than 70 wt %, peroxides are not liable to be decomposed even on addition of a decomposition accelerator as later explained.
There is no particular limitation to the fields of application to which were put the sulfuric acid/peroxide liquid mixtures in use. In general, the sulfuric acid/peroxide liquid mixtures are preferably derived from a variety of rinsing processes. For example, the sulfuric acid/peroxide liquid mixtures used as a rinsing liquid for various parts or products in the electrical field, in particular those used in the production process of semiconductors for resist exfoliation or as a variety of rinsing liquids, are preferred because the sulfuric acid in the waste sulfuric acid/peroxide liquid mixture used in the semiconductor manufacturing process usually has a concentration of 70 wt % or higher.
The peroxides in the sulfuric acid/peroxide liquid mixture may be enumerated by hydrogen peroxide, ozone and peroxo acid (Caro""s acid). It is noted that peroxo sulfuric acid is usually generated by injecting aqueous hydrogen peroxide or ozone gas into sulfuric acid.
There is no particular limitation to the concentration of the peroxides in the sulfuric acid/peroxide liquid mixture. In general, this concentration is preferably 0.001 ppm to 20 wt % and more preferably 0.01 ppm to 10 wt %. There is no particular problem if plural components other than sulfuric acid and peroxides, such as water, inorganic acid, surfactants or decomposition stabilizers are contained in the liquid mixture in addition to sulfuric acid and peroxides. The sum of the contents of these third components is preferably not larger than 30 wt % because the contents of these third components higher than this tends to obstruct decomposition of peroxides in the liquid mixture.
There is no particular limitation to the temperature at the time of decomposition of the peroxides of the sulfuric acid/peroxide liquid mixture. There is raised no particular problem if the decomposition temperature is in a range from 0xc2x0 to 180xc2x0 C. which is the usual using temperature of the sulfuric acid/peroxide liquid mixture. In general, the higher the temperature of the liquid mixture, the faster is the decomposition or foaming speed of the peroxides at the time of decomposition.
It is desirable to add one or more of nitric acid, fuming nitric acid, nitrogen dioxide, nitrate compounds or hydrochloric acid to the sulfuric acid/peroxide liquid mixture as a decomposition accelerator for the peroxides contained therein. There is no particular limitation to the concentration of these compounds which moreover may not as yet be used or already used. From the viewpoint of effective utilization of resources, used compounds are preferred. For the same reason, waste liquids of any one of the aforementioned decomposition accelerators, used as wafer rinsing liquids in the semiconductor manufacturing process, are preferably used as the decomposition accelerators for the present process. The amount of addition of the decomposition accelerators to the sulfuric acid/peroxide liquid mixture can be suitably set depending on the sort or the concentration of the peroxides or the liquid temperature. In general, the amount of addition of the decomposition accelerator to the weight of the liquid mixture is preferably 0.001 ppm to 10 wt % and more preferably 0.01 ppm to 5 wt %. If, when adding the decomposition accelerator, the liquid mixture is stirred sufficiently, the decomposition accelerator can be diffused readily thus assuring an increased decomposition speed or a reduced amount of addition of the decomposition accelerator.
The above-described decomposition reaction can occur at a use point of the liquid mixture, such as at a use position in the wafer rinsing vessel of the semiconductor manufacturing process, or in a device dedicated to the decomposition reaction. Since a large amount of gases, mainly oxygen gases, are usually generated in the present decomposition reaction to yield the heat of decomposition, it is desirable to provide a stirring function for lowering the heat of decomposition and/or a cooling function or to take suitable countermeasures, such as pressure-resistant structures or provision of a pressure releasing line, against rise in liquid surface due to foaming gas or against rise in pressure.
For realizing more effective decomposition, it is advisable to inject the decomposition accelerator dropwise in the stirred-up state of the liquid mixture. This decomposition reaction is generally extremely high in reaction speed, such that, if the decomposition accelerator is injected at a time, the foaming or heat generating reaction tends to be difficult to control.
By the above processing, decomposition of the peroxides in the sulfuric acid/peroxide liquid mixture is completed in a shorter time, such that the processing liquid can be re-used as sulfuric acid. For example, the as-processed liquid can be re-used as a strong acid, or a sulfonating agent. For example, sulfuric acid obtained from the sulfuric acid/peroxide liquid mixture in this manner can be used as a sulfonating agent for the high-molecular compound, or for the hydrolysis reaction of the high-molecular compounds.
An extremely small quantity of the decomposition accelerator can exhibit its effect, so that sulfuric acid in the sulfuric acid/peroxide liquid mixture can hardly be lowered. This significantly improves the regeneration efficiency at the time of purifying the processing liquid following the decomposition to yield sulfuric acid to facilitate regeneration as concentrated sulfuric acid, while lowering the cost.
Moreover, the processing liquid from the decomposition process can be used in the as-decomposed state as sulfuric acid. For example, the as-decomposed processing liquid can be used as a sulfonating agent for various compounds, as sulfonating agents for unsaturated compounds containing aromatic compounds, halogens, alcohols or compounds containing acid anhydride groups, in particular hydrocarbon-based organic compounds, or as hydrolysis accelerators for various compounds. More specifically, the processing liquid can be used for synthesis of alkyl benzene sulfonate (ABS). The above-enumerated compounds can be used as dispersants for a variety of materials, that is surfactants, or as anti-static agents.
The processing liquid can also be reacted with polystyrene to yield polystyrene sulfonates and sodium salts thereof which can be used as a variety of dispersants, for example, as a dispersant for cement or coal-aqueous slurry, as a heat-resistant thermoplastic resin, as an anti-static agent, such as that for paper, resin or fibers, as a flocculent, such as that for processing the effluent, or as paste or ion exchange resins. The processing liquid can also be reacted with lignin to yield lignin sulfonic acid and salts thereof effective as a dispersant for cement. The processing liquid can hydrolyze acrylonitrile groups in the ABS resin, SAN resin or PAN resin to modify the resulting hydrolysates for use as a hygroscopic resin.
The compounds, such as organic compounds, processed with the as-decomposed processing liquid, may be pre-use compounds or used compounds, that is a waste material. From the viewpoint of effective utilization of resources, it is more preferred to use the waste materials as a starting material. For example, a used plastic waste material can be preferably used as a starting material.
The above processing enables the peroxides in the waste liquid of the used sulfuric acid/peroxide liquid mixture to be decomposed efficiently, thus facilitating recovery of the high-concentration sulfuric acid. The recovered sulfuric acid can not only be used on purification thereof as regenerated sulfuric acid but also can be used as sulfonating agent or as hydrolysis accelerator for various compounds, such as various organic compounds.
In the conventional practice, large amounts of water and chemical agents are required for processing the sulfuric acid/peroxide liquid mixture thus ultimately producing a large amount of the waste materials. The amount of the waste material can now be reduced significantly. Moreover, the liquid mixture, so far discarded, can be converted to products with a high added-value. In addition, the plastic waste materials, so far discarded, can similarly be converted to products with a high added-value.
It will be seen from above that, by exploiting the technique according to the present invention, saving in resources or recycling can be realized to reduce the amount of waste material thus contributing to maintenance of global environment.
The present invention will be explained with reference to several preferred Examples which are merely illustrative and are not intended to limit the invention. In the Examples 1 and 2, specified cases of decomposition of peroxides in the waste liquid of the sulfuric acid/peroxide liquid mixture are shown. Similarly, in Examples 3 to 9, specified cases of utilizing decomposition products of the sulfuric acid/peroxide liquid mixture are shown along with Comparative Examples. In the following Examples, in which decomposition of peroxides in the sulfuric acid/peroxide liquid mixture and utilization of a decomposition product thereof are shown, the following waste sulfuric acid/peroxide liquid mixtures (a) and (b) are used for evaluation:
(a) a waste liquid of a sulfuric acid/peroxide liquid mixture used as a rinsing liquid for preprocessing prior to oxide film formation of the semiconductor manufacturing process (pre-step) (initial mixing ratio of H2SO4/H2O=5:1 in volume ratio, use temperature of 80xc2x0 C., with a small quantity of aqueous hydrogen peroxide being supplemented in use); and
(b) a waste liquid of a sulfuric acid/ozone liquid mixture used as a resist liquid for resist exfoliation in the semiconductor manufacturing process (pre-step) (concentrated sulfuric acid with an ozone gas blown therein, using temperature: 110xc2x0).
The original concentration of sulfuric acid and that of aqueous hydrogen oxide are 96 wt % and 30 wt %, respectively.